Question

A photo-cell is illuminated by a small bright source placed 1m away,when the same source of light is placed 2m away,the electrons emitted by photo cathode

• A. Each carry one-quarter of their previous energy
• B. Each carry one-quarter of their previous moments
• C. Are half as numerous.
• D. Are one-quater as numerous.

Answer 1

The correct option is D Are one-quater as numerous.

Number of electrons ejected per second is inversely proportional to the square of the distance between point light source and photocell.
So, $n\propto \frac{1}{d^2}$

$\frac{n_1}{n_2}=\frac{d_2^2}{d_1^2}$

$\frac{n_1}{n_2}=(\frac{2}{1}{)}^2$

$n_2=\frac{n_1}{4}.$ So, number of electrons ejected will be one - quarter of the intial number of electrons ejected.